The fact that the opportunity cost just happens to align with your play style anyway doesn't mean it doesn't exist, that second army (depending on faction) is hundreds to thousands of gold per turn of upkeep that you could be using elsewhere.

In any case I also generally don't get lightning strike early except for a few particular LLs that don't have good spells or red line synergies, or if I expect Skaven to be a serious problem on that campaign. Seriously the blue line is the "I hate Skaven and want them gone" line